The broad, long term objective of the proposed research is the development of Optical Doppler Tomography (ODT) to characterize the intratumoral blood flow at discrete spatial locations within a tumor during Photodynamic Therapy (PDT). The administration of a sufficient dose of light, at the appropriate wavelength, to photosensitizer-containing tissue is essential for PDT. Accordingly, monitoring the effects of light delivery in tissue during PDT has great clinical significance. The specific aims of this proposal are to 1) apply ODT to characterize the velocity distribution of blood flow in preclinical models during PDT; and 2) demonstrate the practical application of the technique to the clinical management of patients with skin cancer receiving PDT.